1. Field of the Invention
The present invention relates to a method for processing sheets of material in register, in particular for making security elements, according to the preamble of claim 1, to an apparatus for carrying out this method, and to sheets of material that can be used as a semifinished product for making security threads.
2. Description of the Related Technology
In order to protect bank notes, papers of value, identity cards, etc., better against forgery, it is known to equip these documents with security threads, in addition to other security features. In the case of paper products the security threads are introduced into the furnish layer as it is forming during paper production, while in the case of multilayer plastic products they are embedded between two or more individual layers.
The security threads are provided with, among other things, printing extending in the longitudinal direction of the thread, whereby such known printing may be present in the form of patterns or alphanumeric characters, optically effective structures and/or readily visual and/or only machine readable prints, additives or coatings. The printing extends in a constant form over the entire length of the thread, whereby a pattern or writing is repeated any number of times. In the following text, a term such as "printed pattern." "printing." etc., stands for any kind of marking: it also includes embossings, punchings, coatings etc.
For reasons of manufacturing technology, printed security threads are produced from wide sheets of film. The sheets of film are first printed with patterns or writing in a parallel arrangement: these sheets of film are then cut into the individual security threads. Since the threads generally have a width of only 0.5-1.5 mm, great effort is usually required to cut the film in register with the printing. One has therefore in many cases preferred to select the individual lines of writing and the width of the threads in such a way that at least one line of writing is always found completely on the thread after it is cut (DE-OS 14 46 851).
Another known method consists in printing the desired pattern on transparent films with large spaces between the prints and then performing the cut in the spaces. After the security thread is embedded in the paper the transparent area is not recognizable: one can only see the printed pattern running in the longitudinal direction of the thread. This method involves the consequence that the thread to be embedded must be considerably wider than the visible thread portion. The embedding of a wide thread has an adverse effect on the quality of the document and security paper, reducing the tearing strength of the paper and the adherence of the thread in the paper. Furthermore, threads exceeding a certain width can no longer be embedded in the paper with the necessary reliability of manufacture, i.e. without forming holes.
A method that avoids the above problems and allows sheets of film to be processed in register is known, for example, from EP-A 0 238 043. In this known method, security threads, or the sheets of film bearing the security threads, are equipped with a mechanically detectable longitudinal surface structure. Using profiled rollers or similar devices which engage these structures, one can thus feed the sheets of film to further processing devices, such as printing devices or cutting devices, in exact alignment with these structures. However, this solution can only be used for sheets of film having a suitable surface structure, or requires an additional method step to apply the surface structure.
Other methods known from general printing technology are to guide sheets of material by means of printed markings. When the sheets of material run into the processing unit, e.g. a cutting unit, the positional deviations of the markings from desired positions are picked up by sensors. A control signal formed therefrom is fed to a register control means which then performs a correction of position (DE-AS 21 46 492). However, this marking-controlled positioning of the sheet involves substantial disadvantages.
Thus, it is necessary to dispose the sensor as close as possible to the actual place of processing in order to avoid sources of error, in particular to prevent the sheets of material from running out between the sensor and the cutting position. Furthermore, the sensor and the processing units must be in a fixed spatial relation to each other, which necessitates an additional stable mechanical connection between the machine and the sensor.
These requirements--a fixed structure and adjacent positioning of the sensor to the processing--lead to many kinds of problems. In many cases the available space does not allow the sensor to be disposed in the immediate vicinity of the place of processing. When this is in fact possible, it causes problems for servicing and adjustment work since the sensors are poorly accessible. Furthermore, the immediate vicinity of the sensitive sensors to the processing device increases the danger of their being soiled and damaged. Also, the required space makes it difficult to retrofit existing cutting machines with a sheet positioning means.
A further serious disadvantage is that the task of precisely supplying the sheet of material can often be fulfilled with sufficient reliability by the known methods only under special operating conditions.
A known arrangement for controlling deviations from a desired position comprises a feeding device (register control means, etc.) and a sensor head which are passed in this order by a sheet of material provided with control markings (DE-AS 21 46 692). The cutting unit is located optionally before or behind the sensor head. The sensor head continuously detects the deviations of the sheet markings from a desired position, forms a control signal and passes it to the feeding device. The feeding device uses the control signal to correct the position of the sheet of material relative to the cutting unit. The disadvantage of this arrangement is that one obtains different control characteristics depending on the momentarily existing parameters of the arrangement (control speed, sensor sensitivity, sheet speed, etc.). If the system damping is too low or, equivalently, if the register control means overreacts, the deviation control means passes into a permanently oscillating state. If the system damping is too high, the time constant of the deviation control is not large enough so that errors are corrected too late. This control means requires precisely fixed parameters and works in the desired manner only within a narrow parameter range. However, the narrower the tolerance limits, the more effort is required for regulation, production methods and production control. At warrantable effort, the attainable cutting tolerances are several tenths of a millimeter.
In the interests of high protection against forgery and reliable embedding in the paper, it is desirable to have narrow, e.g. millimeter-wide, security threads with predefined placement of the marking with respect to the geometry of the thread. The necessary thread dimensions result in a maximum cutting tolerance of 0.1 millimeters. One must take account of the fact that the paper is produced in long sheets and the security threads must accordingly also be made available in long threads. This means that the processing method must guarantee that narrow tolerance limits are met over a long cutting length.
If security threads are cut with a width in the submillimeter range using the known control techniques which correct deviations from the desired position in extremely different ways depending on the existing method parameters, great control effort is required for meeting the tolerances. Large deviations cannot be permitted for the above reasons.
The known control techniques are thus inapplicable, or insufficiently applicable, for making security threads with a printed pattern located exactly over the width of the thread.